Detector and automatic volume control circuit for frequency-modulation receivers



Oct. 5, 1943. c. T. M COY 2,330,902

DETECTOR AND AUTOMATIC VOLUME CONTROL CIRCUIT FOR FREQUENCY MODULATION RECEIVER Filed Oct. 17, 1941 mm. M 1 1.

I 11 mm M 4. M6! VOLT$ Patented Oct. 5, 1943 UNITED STATES PATENT- oFFlcs DETECTOR AND AUTOMATIC VOLUME CON TROL CIRCUIT FOR FREQUENCY-MODU- LATION RECEIVERS Claudius 'r. McCoy, Philadelphia, Pa., assignor to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application October 17, 1941, Serial No. 415,465

17 Claims.

This invention relates to circuits for phaseand frequency-modulation receivers andthe like, and more particularly to a novel combination frequency-detector and automatic-volume-control circuit adapted for use in the reception of frequency-modulated carrier signals.

The present invention is capable of application to FM receivers generally, although it is particularly adapted for use with FM receivers which are not provided with amplitude-limiting devices or circuits. The invention is also particularly adapted for application to switchless FM/AM detector circuits of the type described and claimed in copending application, Serial No. 419,200, filed November 14, 1941.

ume-control voltage from a balanced differen- I tially-connected frequency-detector.

Another object of the invention is to provide means for deriving an AVC voltage from a balanced detector, which voltage varies symmetrically about the center frequency of the detector as thereceiver is tuned to a desired FM signal.

Still another object of the invention is to provide a circuit for deriving an. AVC voltage from a combination FM and AM detector circuit, wherein neither theAVC circuitnor the detector circuit require switching in changing from FM I to AM reception, or vice versa.

Other objects and features of the invention will be apparent from the following description and the accompanying drawing, in which Fig. l is a circuit diagram of a balanced frequency-detector embodying the AVG circuit of the present invention;

Fig. 2 illustrates certain of the characteristics thereof and Fig. 3 is a circuit diagram of a switchless FM/AM detector embodying the novel AVC circuit.

Reference is now made to Fig. -1 in which the invention is applied to a representative type of balanced frequency-detector circuit. It should be understood, however, that the invention is capable of application to balanced detector circuits generally, and not merely to the specific detector circuit illustrated. Well-known-"balanced detector circuits to which the novel AVC circuit may be applied are, for example, those of the. types disclosed in the patents to Round, No. 1,642,173 and to Seeley, No. 2,121,103.

The balanced detector circuit of Fig. 1, insofar as'its operation as a frequency detector is concerned, is more fully described in the abovementioned co-pending application. Briefly, however, it may be stated that the detector per se is constructed, and operates, generally as follows. A pair of diodes, D1 and D2, are each provided with individual load resistors l and I respectively, and with individual untuned windings 3 and 4 respectively. In addition there is provided a tuned winding 5 which is common to the two diodes, the lower end of the winding 5 being coupled to the diode cathodes through the R. F. by-pass-condensers 6 and I, while the upper end of the winding is connected to the diode anodes through the untuned windings 3 and 4, the condenser 8 being of negligible impedance at radio frequencies, as well as at audio frequencies, as will be explained hereinafter. The frequency modulated signal may be supplied to the detector circuit by way of the tuned input winding 9, which in a superheterodyne receiver may be the output circuit of the .receivers intermediate frequency amplifier. Preferably the untuned windings 3 and 4 are tightly coupled to the winding 9, while the tuned winding 5 is coupled loosely thereto. .The tuned windings 5 and 9 may be tuned to the center frequency of the frequency modulated wave supplied by the preceding I. F. amplifier (not shown), and when so tuned the unmodulated signal will give rise to voltages across the untuned windings 3 and 4 which differ in phase from the voltage across the tuned winding '5 by substantially 90 degrees. The diode D1 will, of course, be supplied with an input signal which is the vector sum of the voltages acrossthe windings 3 and 5, while the diode Du will be supplied with a signal which is the vector sum of the voltages across .the windings l and 5. As the frequency of the incoming signal varies, the phase of the voltage across the tuned winding 5 will change with respect, to the voltwinding 5 is tuned.

ages across the untuned windings, and in conse- The rectified output signals appearing across the load resistors I and and 2 are combined differentially by connecting-the anode ends (lefthand ends) of the two load resistors by meansof the by-pass condenser 8. Preferably the impedance of this condenser at the lowest audio frequencies is small compared to the impedance of tector as the associated resistors. so that for audio fre quencies the anode ends ofthe resistors I and 2 may be regarded as joined by a direct electrical connection of negligible impedance. The detected output signal may then be derived from the cathode ends of the resistors I and 2. This differential connection of the two load resistors provides the balanced detector operation which is desired in FM receivers, the rectified audio output of the two diodes being combined in such a manner that the detector is substantially insensitive to amplitude modulation of the carrier wave, since the rectification products of the detection process which correspond to amplitude modulation are combined in opposition in the differential connection of the loads.

According to the present invention the diode loads are connected differentially with respect to the alternating current (audio) output of the detector, but are connected in a relatively reversed (and hence additive) manner with respect to the direct-current (automatic-volumecontrol) output of the diodes. This last-named connection may be made by means of a high resistance I connected between the cathode end of the load resistor I and .the anode end of the load resistor 2. Now when the diodes are supplied'with an I. F. input signal the direct current products of the rectification process will flow through the load resistors in a direction such that the. anode ends thereof assume a negative potential with respect to the cathode ends thereof, as is indicated in the drawing by the.

symbols and and consequently the direct current connection herein described will result in the establishment, between the negative end of theresistor I and the positive end of the resistor 2, of an AVC voltage Whose magnitude is substantially double that which would be derived from either load resistance alone. The alternating or audio components of this voltage maybe substantially removed by means of a filter network comprising, for example, the series resistor II and the shunt audio. by-pass condenser I2.

Inspection of Fig. 1 will disclose that because of the presence of audio by-pass condensers 8 and I2, the resistors I0 and II are effectively in shunt with load resistors I and 2 respectively at audiofrequencies, and hence the audio fre- 7 quency load of the diodes is modified by the presence of the first-named resistors. This effect may be made negligible, however, by assigning resistance values to the elements I0 and II which are high compared to the load resistances I and 2. By making resistances I0 and II equal their effect on the load resistors I and 2 respectively will be substantially equal, although in practice this factor does not appear to be critical. In one satisfactory embodiment of the invention, the diode load resistors I and 2 were assigned values of one-half megohm each, while the resistors Ill and II were each of one megohm.

The present invention, in addition to providing an AVC voltage which is substantially double that supplied by either diode singly, provides an AVC voltage whose magnitude varies symmetrically about the center frequency of the dethe receiver is tuned to a desired FM signal. This may best be understood by referring to Fig. 2 in which the curves A and B represent, for the diodes Di' and D2 respectively,

represented by the curves A and B would be in opposition so that at the center frequency, 1%, of

the balanced detector the overall D. C. output would be zero, the output being positive on one side of ,fc and negative on the other side thereof. By the connection illustrated in Fig. l, the D. C. outputs are additively combined to produce the overall D. C. output characteristic represented by the curve C. This curve, it will be observed, is substantially symmetrical about the center frequency fc. This symmetrical AVC characteristic is of particular value in FM receivers which are not equipped with some form of tuning aid to indicate when the receiver is properly tuned to the desired station. By employing the symmetrical AVC characteristic C, the volume and quality of the selected transmission will vary symmetrically on either side of the proper tuning frequency fc and this will enable the operator to judge the proper setting more accurately than if the AVG characteristic were unsymmetrical about fc as are the curves A and B.

direct current output. (AVC volts) against car- Reference is now made to Fig. 3 wherein it is shown how the above-described AVC circuit may be combined with the switchless AM/FM detector described and claimed in the copending application hereinbefore referred to. Corresponding elements in Figs. 1 and 3 are similarly numbered, and as in Fig. 1 the untuned windings 3 and 4 are tightly coupled to the primary winding 9, while the-tuned winding 5, which is common to the input circuits of both diodes, is loosely coupled to the winding 9. The added elements, which comprise the tuned and coupled windings I3 and I4, make it possible to utilize certain of the elements I to I2 inclusive in detection, and in the provision of an AVC voltage, in both the FM and AM bands. As in Fig. 1 the windings 5 and 9 are tuned to the intermediate frequency employed in FM reception, commonly 4.3 megacycles, while the windings I3 and H may be tuned to the intermediate frequency employed in AM reception, commonly 455 kilocycles. As is disclosed in detail in the above-mentioned copend- 7 ing application, the radio frequency and intermediate frequency stages of the preceding receiving circuits may be constructed and arranged to supply either a frequency-modulated 4.3 megacycle signal or an amplitude-modulated 455 kilocycle signal to the detector input terminals I5. If the former signal is supplied the signal will be applied to the diodes by way of the windings 9, 3, I, and 5, no substantial voltage appearing across the windings I3 and I4 which are tuned to a frequency differing substantially from that impressed on the terminals I5. Because the impedance of the condenser across the 455 kilocycle winding I4 is negligible at 4.3 megacycles,

the series insertion of this tuned circuit between the upper end of the winding 4 and the junction of resistors 2 and III will have no appreciable effect upon the operation of the circuit as a detector of the frequency-modulated 4.3 megacycle signal. However if an amplitude-modulated 455 kilocycles signal is supplied to the terminals I5,

negligibly small and'hence these windings will,

exert no appreciable effect upon the operation of Y the circuit. The amplitude-modulated 455 kilocycle signal appearing across the tuned second arywinding M will be applied to the diode D: as follows, from the lower terminal of the winding I4, through the winding (of negligible impedance at 455 kc.), and through the I. F. by-pass condenser 6, to the cathode'of' D2; and from the upper terminal of the winding I4, through the winding 4 (of negligible impedance at 455 kc.), t0 the anode of D2. No appreciable 455 kc. signal will be applied to the diode Di, and hence this diode will have no substantial effect under these conditions.

During 455 kc. (AM) operation, AVC voltage may be derived from the negative (anode) end of the load resistance 2 by way of the serially connected resistors I0, I, and H, the control voltage appearing at the junction of condenser l2 and resistor I l. The audio output signal may again be derived from the junction of resistors I and I0 and the cathode-of D1, this point being coupled to the ungrounded end of the resistor 2 throughthe resistors i and ill, the resistors I and Ill being effectively in parallel for audio frequency currents since the impedance of the condenser 8 is negligible at these frequencies.

Thus in the novel combination illustrated in Fig. 3 there is provided a switchless circuit adapted for the balanced detection of a frequency-modulated signal of one carrier frequency, or for the single-diode detection of an amplitude-modulated signal of a different carrier frequency, while simultaneously providing an AVC voltage of desirable characteristics irrespective of the form of the input signal.-

Although the invention has been described I with particular reference to radio receivers of the superheterodyne type and to the embodiments illustrated in the drawing, it will be understood that the invention is capable of general appli-.

cation and other forms of physical expression, and is, therefore, not to be limited to the speciflc disclosure, but only by the scope of the appended claims.

I claim:

1. In a frequency-modulation receiver, a balanced detector circuit comprising a pair of vacuum tubes, 9. load resistor connected to each of said tubes, conductive means having a high impedance at audio frequencies for connecting said load resistors in a series-aiding relation with respect to the direct-current components of voltage across said resistors, thereby to provide an additive automatic volumecontrol circuit, and an audio output circuit including capacitive means having a relatively low impedance at audio frequencies for connecting said load resistors in a relatively reversed relation, thereby to provide a difierential audio output circuit.

2. In a frequency modulation receiver, a balanced detector circuit comprising a pair of diodes, an input circuit for supplying a radio frequency signal to said diodes, a load resistor connected in the anode-cathode circuit of each of said diodes, there being audio and direct current components of voltage established across each of said detector load resistors, an automatic volume control circuit including conductive means having a high impedance at audio freassaeoa 3. A combination frequency-detector and automatic-volume-control circuit for frequency modulation receivers and the like, comprising a-pair of diodes provided with a discriminator-type input circuit, a load resistor for each of said diodes, means including an alternating-current coupling element for connecting said load resistors in a predeterminedseries relation for alternating currents, and means including a directcurrent coupling element for connectingsaid load resistors in a relatively reversed series relation for direct currents.

4. Acombination frequency-detector and automatic-volume-control circuit for frequency modulation receivers and the like, comprising a pair of diodes provided with a discriminatortype input circuit, a load resistor for each of said diodes, an audio by-pass condenser, a coupling resistor having a resistance greater than that of either of said load resistors, means including said condenser for connecting said load resistors in a predetermined series relation for alternating a first load resistor connected between cathode and anode of one of said diodes, a second load resistor connected between cathode andanode of the other of said diodes, a high resistance connected between the cathode' end of said first resistor and the anode end of said second resistor, an impedance having high impedance to direct current and low impedance to audio frequency currents connected between the anode ends of said resistors, an audio output circuit connected to the cathodes of said diodes, and a circuit for deriving a gain controrvoltage fromsaiddetector connected between the cathode end of said'second resistor and the anodeend of said first resistor. a

6. A combination frequency-detector and automatic-volume-control circuit for frequency modulation receivers, comprisin a pair of space discharge devices each having first and secondelectrodes, a discriminator type input circuit connected to said devices, a first load resistor connected between the first and second electrodes of one of said devices, a second load resistor connected between the first and second electrodes of the other of said devices, a high resistance connected between the first-electrodeend of said first resistor and the second-electrode end of said second resistor, an audio by-pass condenser connected between the second-electrode ends of said resistors, an audio output circuit connected to said first electrodes, and a circuit for deriving a gain control voltage from said detector connected between the first electrodeend of said second resistor .and the second-electrode end of said first resistor.

v 7. A combination frequency-detector and automatic-'volume-control circuit for frequency modulation receivers as claimed in claim 6, characterized in the provision of a radio frequency by-pass condenserconnected between the first electrodes of said space discharge devices.

8. A combination frequency-detector and automatic-volume-control circuit, comprising a pair of diodes, radio frequency input circuits for said diodes, said circuits being constructed and arranged to supply out-of-phase and unequal radio frequency voltages to said diodes in response to a frequency-modulated input signal, a load impedance connected to each of said diodes, an audio by-pass condenser for interconnecting said load impedances in a predetermined series relation in an audio output circuit, and an audio blocking resistor for interconnecting said load impedances in a relatively reversed series relation in an automatic volume control circuit.

9. A combination frequency-detector and automatic-volume-control circuit, comprising a pair of diodes, an untuned input circuit winding connected to each of said diodes, a tuned input circuit winding connected in a circuit which is common to both of said diodes, a load resistor for each of said diodes,. means including an impedance element for connecting said load resistors in a predetermined series relation in an alternating-current output circuit, and means including another'impedance element for connecting said load resistors in a relatively reversed series relation in an automatic volume control circuit.

10. A combination frequency-detector and automatic-volume-control circuit for frequency modulation receivers, comprising a pair of diodes, a discriminator type input circuit for supplying said diodes with a frequency-modulated highfrequency signal, a load resistor in the anodecathode circuit of each of said diodes, a condenser of relatively high capacity for connecting said diode loads differentially with respect to the rectified output of said diodes, thereby to provide a low frequency output which is substantially double the low frequency output of either of said diodes, and a high impedance conductive element for connecting said diode loads additively with respect to the rectified output of said diodes, thereby to provide a direct current voltage which is greater than the direct current voltage of either of said diodes.

11. A detector and automatic-volume-control circuit for radio receivers, comprising a first vacuum tube and a first load impedance therefor, a second vacuum tube and a second load impedance therefor, a radio frequency input circuit for applying a modulated carrier signal to said vacuum tubes, 9. pair of audiooutput terminals, a pair of direct current output terminals,

means including an alternating-current coupling element for connecting said load resistors in a predetermined series relation between said audio output terminals, and means including a directcurrent coupling element for connecting said load resistors in a relatively reversed series relation between said direct current output terminals.

12. A detector and automatic-volume-control circuit as claimed in claim 11, characterized in that said coupling elements comprise an audio by-pass condenser and a high resistance, respectively.

13. A combination switchless detector and automatic-volume-control circuit for use in frequency-and-amplitude modulation receivers comprising a pair of diodes provided with a discriminator type input circuit, said input circuit being tuned to a first intermediate frequency, a load resistor for each of said diodes, a second input circuit tuned to a second and substantially different intermediate frequency, said second tuned circuit being operatively connected to only one of said diodes, frequency selective means for supply- I ,capable of receiving both frequency-modulation and amplitude-modulation signals, a pair of diodes, a load resistor connected to each of said diodes, means for supplying a frequency-modulation signal of predetermined frequency to each of said diodes, said diodes being interconnected to provide balanced detection of said frequencymodulation signal, means for supplying an amplitude-modulation signal of different predetermined frequency to one of said diodes, fixed circuit means for deriving an automatic-volumecontrol voltage from said one diode during amplitude-modulation operation, and fixed circuit means for deriving a symmetrical automaticvolume-control voltage from said pair of diodes during frequency modulation operation.

15. In a detector and automatic-volume-control circuit adapted to operate selectively upon signals of predetermined and substantially different characteristics, a pair of diodes, a pair of load impedances therefor, a discriminator type input circuit coupled to said diodes, means for supplying a frequency-modulated signal to said discriminator circuit, a single input circuit, connected to one of said diodes, means for supplying anamplitude-modulated signal to said single input circuit, connections between said load impedances for deriving a symmetrical automaticvolume-control voltage from both of said diodes during the reception of a frequency-modulated signal, and connections for deriving an automatic-volume-control voltage from one of said diodes during the reception of an amplitudemodulated signal.

16. In a radio receiver, a pair of space discharge devices, a pair of output impedances therefor, frequency-selective fixed circuit means connected to said devices and to said impedances for alternatively detecting amplitude and frequency modulation signals, means including both of said devices and both of said impedances for generating and supplying a symmetrical automatic-volume-control voltage in response to a frequency-modulated signal, and means including only one of said devices and one of said impedances for generating an automatic-volumecontrol voltage in response to an amplitude-modulated signal.

17. In a frequency-modulation receiver, a balanced frequency detector circuit including a pair of impedances across which direct-current voltages appear as a result of the detection process,

gain thereof.

CLAUDIUS T. MCCOY. 

